This invention relates to an improved multisegmented electrophoretic image display cell or an EPID cell. EPID cells are known in the art and are described for example in Ota, U.S. Pat. No. 3,792,308 and Ota et al, Proceedings of the IREE Vol. 61, No. 7, July 1973, pages 832-836.
An EPID cell of the type employed in the invention is composed of charged light reflecting pigment particles suspended in a dark colored liquid sandwiched between a front, transparent electrode and a back electrode, patterned into segments, and a field or background electrode.
By applying a D.C. field across the suspension, the particles are moved to one or the other of the electrodes depending upon the polarity of the charged particles.
For example, in the area of negatively charged pigments when the front transparent electrode is at ground potential, the field electrode is positive, the selected segment electrodes are negative and the non-selected segment electrodes are positive, the particles in the area of the selected segment electrodes are repelled to the front transparent electrode while the other particles in the area of the positively charged field and non-selected segment electrodes are attracted to those electrodes. The observer, viewing through the transparent electrode, sees the shape of the selected segmented electrodes due to the deposited layer of light reflecting particles on a dark background, since the dark suspension liquid hides the pigment attracted to the field electrode and the non-selected segment electrodes. By switching the polarity so that the selected electrode segments are positive and the field and non-selected segment electrodes are negative, the position of the pigment is reversed so that the observer now sees the selected electrode segment as a dark image on a light background.
A problem with these EPID devices arises from the fact that lead lines conducting current from the power source to the electrode segments themselves act as electrodes and tend to attract or repell the charged pigment and thus become visible to the observer and detract from the appearance of the desired image.
In order to hide these lead lines, cut-out masks have been placed between the observer and the display or the part of the transparent electrode facing the lead lines is painted over with an opaque paint.
These methods have not proven to be too successful, as the difficulty of matching the color of the mask or paint to the color of the suspension liquid often detracts from the appearance of the EPID cell, particularly when employed in such a consumer item as a clock.
Another problem is that the use of masks or opaque paints is not possible in those EPID cells where it is desired to reverse the tone of the background and the segmented electrodes by reversing their polarity.